This invention relates to control systems for hydraulic circuits, and more particularly it is concerned with a control system for a hydraulic circuit having externally operated on-off valves interposed between the control valve and the actuator for allowing and blocking a flow of hydraulic fluid therebetween, in which the control valve and on-off valves are actuated and switched in accordance with an operation signal produced by an operation device to thereby control the speed of operation of the actuator in accordance with the flow rate of the hydraulic fluid flowing through the control valve.
In one type of hydraulic circuit in which the speed of operation of the actuator is controlled, externally operated on-off valves are mounted between the control valve and actuator, as disclosed in JP, A, No. 57-154505, for example.
The on-off valves of this type of hydraulic circuit are mounted to avoid a fall of the driven article which might otherwise occur due to damage to the piping of the circuit, etc. In one manner of operation of this type of hydraulic circuit, the control valve is operated after the on-off valves are opened, so that a hydraulic fluid is supplied from a fluid source to the actuator to accelerate the operation of the actuator. In this type of hydraulic circuit of the prior art, if the on-off valves are opened to bring an inlet port of each on-off valve into communication with its outlet port when there is a difference in pressure between the upstream and downstream of the valve, the hydraulic fluid would flow from the higher pressure side to the lower pressure side as soon as the valve is opened, thereby applying an impact of shock to the actuator. Thus, even if the control valve is actuated slowly to accelerate the load gradually by slowly increasing the speed of operation of the actuator, the load would vibrate due to its inertia and the spring effect of the hydraulic fluid in the piping of the circuit as the impact of shock is applied to the actuator.